1. Field of the Invention and Related Art Statement
The present invention relates to a digital modulator.
Thus far, modulators for modulating carriers using digital signals in the base band and for obtaining IF (RF) band modulation signals use an analog signal process.
FIG. 12 is a view showing a general structure of an orthogonal modulator for processing analog signals.
Digital input signals which are input from an I channel data input point I and a Q channel data input point 2 are converted into analog signals by D/A converters 4 and 5 via digital signal processing section 3, respectively. After that, the signals pass through (operational) amplifiers 6 and 7 and low pass filters 8 and 9, respectively. Then, a component of the signals, whose phase is the same as a reference carrier 12, and another component of the signals, whose phase is orthogonal to the reference carrier 12 via a phase shifting device 13 are modulated by multipliers 10 and 11, respectively. These output signals are mixed by an adder 14 and an orthogonally modulated wave is obtained.
However, when the orthogonal modulation is conducted with the orthogonal modulator for processing analog signals, there are many adjustment points.
First, when an input digital signal is converted into an analog signal by the D/A converters 4 and 5, an DC offset takes place. In other words, although the input digital signal should be a waveform whose amplitude is based on 0 V, the waveform varies from a particular DC level due to the nature of the D/A converters 4 and 5. When such a DC offset is present, the DC component also causes the carrier to be modulated and thereby a correct modulation wave cannot be obtained. Thus, in the modulator described above the operational amplifiers 6 and 7 are disposed downstream of the D/A converters 4 and 5. By adjusting the operational amplifiers 6 and 7, the DC offset is removed.
Second, unless the amplification of the I channel signal which is input to the adder 14 is balanced with that of the Q channel signal, a correct orthogonal modulation wave cannot be obtained. In other words the amplitude of the I channel signal which passes through the low pass filter 8 should accord with that of the I channel signal which passes through the low pass filter 9. However, each amplitude is deflected by individual fluctuation of the characteristics of the D/A converters 4 and 5 and the low pass filters 6 and 7. Thus, in such a modulator, to balance the amplitudes, the operational amplifiers 6 and 7 downstream of the D/A converters 4 and 5 are adjusted.
Third, the input impedance of the I channel of the digital input signal should be precisely matched with that of the Q channel thereof. Thus, when the circuit is made, these impedances should be adjusted.
In the orthogonal modulator for processing an analog signal, there are many points to adjust and thereby much adjustment work is required. Thus, the cost of the modulator becomes expensive.
In addition, for adjusting each input impedance of the I channel and the Q channel of the digital input signal, an impedance matching circuit which consumes large power is required and thereby the modulator becomes a large power consumption type.